Traction performance is one of the primary evaluation criteria for tire treads, and performance on wet surfaces such as snow and ice is an important factor in that evaluation.
Deformation of tread rubber induced by road surface asperities, rate of water drainage between the tread and road surface, and possible adhesive interactions at the interface between tread and road are some of the complex, interrelated factors that complicate the type of quantitative mechanistic understanding needed to formulate tread compounds. To further improve tire performance, those involved in tread design and manufacture continue to investigate the numerous factors that affect wet traction.
Rubber goods such as tire treads are made from elastomeric compositions that contain one or more reinforcing materials; see, e.g., The Vanderbilt Rubber Handbook, 13th ed. (1990), pp. 603-04. The first material commonly used as a filler was carbon black, which imparts good reinforcing properties and excellent wear resistance to rubber compositions. However, carbon black-containing formulations often suffer from increased rolling resistance which correlates with an increase in hysteresis and heat build-up during operation of the tire, properties which need to be minimized to increase motor vehicle fuel efficiency.
The increased hysteresis resulting from the use of carbon black can be somewhat counteracted by reducing the amount (i.e., volume) of and/or increasing the particle size of the carbon black particles, but the risks of deterioration in reinforcing properties and wear resistance limits the extent to which these routes can be pursued.
Over the last several decades, the use of amorphous silica and treated variants thereof, both alone and in combination with carbon black has grown significantly. Use of silica fillers can result in tires with reduced rolling resistance, increased traction on wet surfaces, and other enhanced properties.
Despite the outstanding performance of treads employing silica and carbon black as reinforcing fillers, ever-more demanding regulatory and performance demands have led to continued investigations of alternative fillers. Examples of such non-conventional fillers include aluminum hydroxide (see, e.g., U.S. Pat. Nos. 6,242,522 and 6,489,389 as well as H. Mouri et al., “Improved Tire Wet Traction Through the Use of Mineral Fillers,” Rubber Chem. and Tech., vol. 72, pp. 960-68 (1999)); metal oxides having very high densities (see U.S. Pat. No. 6,734,235); magnetizable particles such as iron oxide or strontium ferrite used in the manufacture of tire sidewalls (see U.S. Pat. No. 6,476,110); macroscopic (e.g., 10-5000 μm mean diameter) particles of hard minerals such as alumina, CaCO3, and quartz (see U.S. Pat. No. 5,066,702); pumice containing SiO2 (U.S. Publ. No. 2004/0242750 A1); sub-micron ZnO particles (see U.S. Pat. No. 6,972,307); and micron-scale ZnSO4, BaSO4 and/or TiO2 (see U.S. Pat. No. 6,852,785). More often, potentially useful fillers are merely strung together in list format; see, e.g., U.S. Pat. Nos. 4,255,296 and 4,468,496. Other non conventional filler materials include clays and complex oxides.
Recently, replacing some or all of the more common types of particulate fillers with inorganic oxides such as ferric oxide, ferrous oxide, aluminum oxide, etc., has been shown to provide vulcanizates with superior wet traction properties; see, e.g., U.S. Publ. No. 2008/0161467).
Enhancing dispersion of reinforcing filler(s) throughout the elastomer(s) can improve processability and certain physical properties. Efforts in this regard include high temperature mixing in the presence of selectively reactive promoters, surface oxidation of compounding materials, surface grafting, and chemically modifying the polymer(s).
Chemical modification of polymers often occurs at a terminus. Terminal chemical modification can occur by reaction of a terminally active, i.e., living (i.e., anionically initiated) or pseudo-living, polymer with a functional terminating agent. Terminal modification also can be provided by means of a functional initiator, in isolation or in combination with functional termination. Functional initiators typically are organolithium compounds that additionally include other functionality, typically functionality that includes a nitrogen atom. Unfortunately, functional initiators generally have relatively poor solubility in hydrocarbon solvents of the type commonly used in anionic polymerizations and cannot maintain propagation of living ends as well as more common alkyllithium initiators such as butyllithium; both characteristics negatively impact polymerization rate and efficiency.
Polymers incorporating 3,4-dihydroxyphenylalanine (DOPA) have been synthesized for some time, often for adhesive applications; see, e.g., U.S. Pat. No. 4,908,404. Because these polymers can be costly and difficult to produce, so-called bulk polymers approximating their performance have been pursued; see Westwood et al., “Simplified Polymer Mimics of Cross-Linking Adhesive Proteins,” Macromolecules 2007, 40, 3960-64. However, the de-protection step utilized by the foregoing approach cannot be used when the polymer contains ethylenic unsaturation.